Switching power supplies are widely used to provide regulated voltages and currents to circuit loads. A switching power supply converts an input voltage by temporarily storing energy corresponding to the input voltage and then releasing that energy to the load, so as to obtain a DC output voltage. The storage of energy (carried out with either magnetic components, like inductors and transformers, or capacitors) is controlled by means of a switching element (such as a power MOSFET).
Different circuit topologies are known within the class of the switching power supplies. A well-known topology is that referred to as “flyback topology”. As known to those skilled in the art, a switching power supply having the flyback topology, also referred to as “flyback converter”, may be directly connected to a main (AC) voltage source, and its (DC) input voltage is obtained by rectifying and filtering the main voltage by means of an input network including a rectifier bridge and a capacitive filter. A flyback converter includes an electric transformer, with the switching element that periodically connects a primary winding of said transformer to the input network (providing the input voltage), in such a way to modulate the energy that is transferred to a secondary winding coupled to the load.
Generally, the flyback converter, like all the switching power supplies, requires a control circuit that carries out the task of properly driving the switching element.
The simplest control circuits of such type—herein referred to as “constant frequency (CF) control circuits”—are designed for driving the switching element with a constant frequency. In this case, the control circuit has a simple circuit architecture, including for example an oscillator and a flip-flop that controls a driver connected to the switching element. Thus, the switching element is opened and closed according to the frequency of the oscillator; by regulating the frequency of the oscillator, it is possible to regulate the transfer of energy, and, thus, the level of the output voltage. The CF control circuit has the advantage of allowing the flyback converter to work according to two different modes.
Particularly, the flyback converter may work in the so-called Continuous Conduction Mode (CCM) or in the so-called Discontinuous Conduction Mode (DCM), depending on the behavior of a current flowing through the secondary winding during the switching cycle of the switching element: in the CCM, the current flows through the secondary winding for the whole period occurring between the opening of the switching element and the closure thereof. Conversely, in the DCM, the current in the secondary winding diminishes so as to reach a null value before the ending of said period.
The DCM is usually exploited when the powers to be managed are low, since in this case it is possible to reduce the sizes of the magnetic components of the converter. For higher powers the CCM is instead preferable, since it allows obtaining a more advantageous current's shape factor.
A more complex family of control circuits for flyback converters consists of control circuits—herein referred to as “Quasi-Resonant (QR) control circuits”—designed for driving the switching element with a frequency that varies over time, depending on the magnetization status of the transformer (which in turn depends on the input voltage and the output current). Particularly, the QR control circuit synchronizes, with a proper delay, the switching of the switching element with the instant at which the current flowing in the secondary winding reaches a null value, in such a way that the switching element is activated when a voltage at a terminal of the switching element connected to the primary winding is near zero. The latter condition is also known as Zero Voltage Switching (ZVS) condition.
Examining in greater detail the behavior of the switching element during the operation of the flyback converter, the voltage at the terminal of the switching element connected to the primary winding—herein referred to as “switching voltage”—starts to oscillate with damped oscillations immediately after the instant at which the current flowing in the secondary winding reaches the null value. In this way, the switching voltage exhibits a sequence of falling edges followed by corresponding rising edges, which determine a corresponding sequence of minimum values, each one representing a possible ZVS conditions.
Since the switching element driven by the QR control circuit is activated very near the instant at which the current of the secondary winding has reached the null value, the operation of the flyback converter corresponds to the boundary between the CCM and DCM. Usually, a flyback converter driven by the QR control circuit has to work with powers under 100 Watts.
Compared to the CF control circuit, the QR control circuit has several advantages.
The most important advantage regards the electromagnetic compatibility. Indeed, being the working frequency variable depending on the input voltage and the output current, the energy irradiated by the converter belongs to a plurality of energy bands. In this way, instead of being concentrated at a single frequency with a high intensity, the electromagnetic emission is distributed among different energy bands with much lower intensities.
Another significant advantage given by the utilization of the QR control circuit regards the possibility of switching the switching element in the ZVS condition. Indeed, since the switching element is usually a MOSFET connected between the primary winding and a ground terminal, in the ZVS condition the voltage across the MOSFET is near to zero, and thus the capacitive losses due to its drain-to-source parasitic capacitance are negligible.
In the variable-frequency operation of the flyback converter, the switching frequency increases when the output load is low ant the input voltage is high. This increase can drastically increment the losses due to parasitic elements of the flyback converter.
This situation is usually addressed by introducing a circuit (implementing a so-called valley skipping technique) that sets a minimum period during which the switching element has to remain activated. In this way, if the ZVS condition occurs before the end of the minimum period, the ZVS condition is ignored, and the switching element remains deactivated. More particularly, the switching element is activated at the occurrence of the ZVS condition corresponding to the first minimum value—in the sequence determined by the oscillations of the switching voltage—after the expiration of the minimum period.
At the present time, the choice of which type of control circuit to be used—between the CF type and the QR type—depends on several and different factors, like for example the personal skill of a technician, the complexity and the cost of the circuit and the amount of power that has to be transferred.
More complex structures of the control circuit have also been proposed.
An example of a control circuit for flyback converters known in the art is the device TEA1654 by Philips. This device is a QR control circuit that normally operates at a variable switching frequency. However, if the switching frequency exceeds a predetermined value, the switching frequency is locked to the value and the device operates in the CF mode. This solution has the great drawback that in the latter condition the switching element may be activated when the switching voltage has a value that is significantly higher than zero (with significant capacitive losses).
Another control circuit for flyback converters known in the art is the device TDA16846/TDA16847 by Infineon. This device is again a QR control circuit that operates at a variable switching frequency. Moreover, the device is capable of always operating in the QR mode, but in such a way to control the switching element with a switching frequency that is as close as possible to a predetermined value. However, since the device always drives the switching element following the occurrences of the ZVS condition, it is not possible to use it for controlling the flyback converter in the CCM.